Methods and apparatus for unified significance map coding

ABSTRACT

Methods and apparatus are provided for unified significance map coding. An apparatus includes a video encoder (400) for encoding transform coefficients for at least a portion of a picture. The transform coefficients are obtained using a plurality of transforms. One or more context sharing maps are generated for the transform coefficients based on a unified rule. The one or more context sharing maps are for providing at least one context that is shared among at least some of the transform coefficients obtained from at least two different ones of the plurality of transforms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/333,808, filed on May 12, 2010, which is incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present principles relate generally to video encoding and decodingand, more particularly, to methods and apparatus for unifiedsignificance map coding.

BACKGROUND

To exploit the non-stationary characteristics of input video content, avideo encoder relies on an entropy coding to map an input video signalto a bitstream of variable length-coded syntax elements.Frequently-occurring symbols are represented with short codewords whileless common symbols are represented with long codewords.

The International Organization for Standardization/InternationalElectrotechnical Commission (ISO/IEC) Moving Picture Experts Group-4(MPEG-4) Part 10 Advanced Video Coding (AVC) Standard/InternationalTelecommunication Union, Telecommunication Sector (ITU-T) H.264Recommendation (hereinafter the “MPEG-4 AVC Standard”) supports twoentropy coding methods. In particular, symbols are coded using eithervariable-length codes (VLCs) or context-adaptive arithmetic coding(CABAC) depending on the entropy encoding mode. Using CABAC, the processof coding a data symbol includes the following three elementary steps:

-   -   1. Binarization: In the binarization step, a given non-binary        valued syntax element is uniquely mapped to a binary sequence,        called a bin string. This process is similar to the process of        converting a symbol into a variable-length code but the binary        code is further encoded.    -   2. Context modeling: A “context model” is a probability model        for one or more bins of the bin strings, and is chosen from a        selection of available models depending on the statistics of        recently-coded data symbols. The context model stores the        probability of each bin being a “1” or “0”, and it is updated        based on the actual coded value.    -   3. Binary arithmetic coding: An arithmetic coder encodes each        bin according to the selected probability model.

In the MPEG-4 AVC Standard, context models and binarization schemes foreach syntax element are defined in the standard. The context model ofeach bin is identified by a context index γ and each probability modelrelated to a given context index γ is determined by a pair of twovalues, namely a probability state index σ_(y) and the (binary) value ω_(γ) of the most probable symbol (MPS).

Suppose a pre-defined set of past symbols, called a context template T,and a related set C={0, . . . , C−1} of contexts is given, where thecontexts are specified by a modeling function F:T→C operating on thetemplate T. For each symbol x to be coded, a conditional probabilityp(x|F(z)) is estimated by switching between different probability modelsaccording to the already coded neighboring symbols z∈T. After encoding xusing the estimated conditional probability p(x|F(z)), the probabilitymodel is updated with the value of the encoded symbol x. Thus, p(x|F(z))is estimated on the fly by tracking the actual source statistics. Toreduce the model cost and avoid inaccurate estimates of p(x|F(z)) due toa large number of C, the MPEG-4 AVC Standard puts two restrictions onthe choice of the context models. First, very limited context templatesT consisting of a few neighbors of the current symbol to encode areemployed. Second, context modeling is restricted to the selected bins ofthe binarized symbols.

At the beginning of each coded slice, the context models are initializeddepending on the initial value of the Quantization Parameter (QP) sincethe quantization parameter has a significant effect on the probabilityof occurrence of the various data symbols.

CABAC Entropy Coding of Residual Data in the MPEG-4 AVC Standard

For the CABAC coding of the residual data in accordance with the MPEG-4AVC Standard, the syntax elements and their related coding scheme arecharacterized by the following distinct features:

-   -   A one-bit symbol coded_block_flag and a binary-valued        significance map are used to indicate the occurrence and the        location of nonzero transform coefficients (namely, significant        coefficients) in a given block.    -   Non-zero levels are encoded in the reverse scanning order.    -   Context models for coding of nonzero transform coefficients are        chosen based on the number of previously transmitted nonzero        levels within the reverse scanning path.

Turning to FIG. 1, an example of the significance map encoding procedurein accordance with the MPEG-4 AVC Standard is indicated generally by thereference numeral 100. The procedure 100 includes a start block 110 thatpasses control to a function block 120. The function block 120 encodes asyntax element coded_block_flag, and passes control to a decision block130. The decision block 130 determines whether or not coded_block_flagis equal to one. If so, then control is passed to a function 140.Otherwise, control is passed to an end block 199. The function block 140performs steps relating to encoding the significance map, and passescontrol to a function block 150. The function block 150 performs stepsrelating to encoding level information, and passes control to the endblock 199. Thus, regarding decision block 130, if the coded_block_flagindicates that a block has significant coefficients, then abinary-valued significance map is encoded by function block 140. Foreach coefficient in the scanning order, a one-bit symbolsignificant_coeff_flag is transmitted by function block 140. If thesignificant_coeff_flag symbol is equal to one, i.e., if a nonzerocoefficient exists at this scanning position, then a further one-bitsymbol last_significant_coeff_flag is sent by function block 140. Thissymbol last_significant_coeff_flag indicates if the current significantcoefficient is the last one inside the block or if further significantcoefficients follow.

When encoding the significance map of the transform coefficients, thechoice of context models of significant_coeff_flag andlast_signficant_coeff_flag depend on the scanning position. In theMPEG-4 AVC Standard, for 4×4 or smaller transform sizes, a context modelis assigned to significant_coeff_flag and last_significant_coeff_flagfor each position, respectively. For the 8×8 transform size and larger,a few transform coefficient positions share one context model in orderto reduce the number of context models.

Significance Map Coding in KTA

The video coding experts group (VCEG) “key technical area” (KTA)software has provided a common platform to integrate the new advances invideo coding after the MPEG-4 AVC Standard is finalized. Proposals touse extended block sizes and large transforms were adopted into KTA. Inthe current KTA software, motion partitions larger than 16×16 pixels areimplemented. In particular, macroblocks of sizes 64×64, 64×32, 32×64,32×32, 32×16, 16×32 are used in addition to the existing MPEG-4 AVCStandard partitioning sizes. Larger block transforms are also used tobetter capture the smoother content in the high-definition video. Suchlarger block transforms include those having sizes of 16×16, 16×8, and8×16. To keep the number of context models low, 8×16, 16×8, and 16×16transforms all use 15 or fewer context models for significant_coeff_flagor last_significant_coeff_flag. Turning to FIG. 2, an example of using15 contexts for a syntax element significant_coeff_flag for an 8×8 blockis indicated generally by the reference numeral 200. In further detail,example 200 illustrates how multiple transform coefficient positions ina block share one context model when an 8×8 transform is used forsignificant_coeff_flag. Each different number represents a contextmodel. When a number is repeated at multiple positions, these positionsshare one context model. In this approach, how multiple transformcoefficient positions share one context, denoted as context sharing, isdesigned for each transform size. The exact pattern of context sharingis denoted as the context sharing map.

Turning to FIG. 3, an example of using 15 contexts for a syntax elementsignificant_coeff_flag for a 16×16 block is indicated generally by thereference numeral 300. In further detail, example 300 illustrate howmultiple transform coefficient positions in a block share one contextmodel when a 16×16 transform is used for significant_coeff_flag. Thepattern for context sharing of 16×16 transform is approximately anupsampled version of that of 8×8. However, this may disadvantageouslyfail to capture the difference in coefficient distributions of differenttransforms.

Significance Map Coding in a Particular Prior Art Approach

In a particular prior art approach, a new context modeling approach wasproposed for 8×8 transform sizes and larger. To model the contexts forthe syntax element significant_coeff_flag for 8×8 blocks, the transformblock is decomposed into 16 sub-blocks of 2×2 samples, and each of thesesub-blocks is associated with a separate context. The context modelselection for larger transform blocks (e.g., for blocks greater than8×8) is based on the number of already coded significant transformcoefficients in a predefined neighborhood (inside the transform block).For coding of the last_significant_coeff_flag, a context modeling hasbeen designed that depends on a distance measure of the current scanposition to the top-left corner of the given transform block. To be morespecific, the context model for coding the last_significant_coeff_flagis chosen based on the scan diagonal on which the current scanningposition lies (i.e., it is chosen based on x+y, where x and y representthe horizontal and vertical location of a scanning position inside thetransform block, respectively). To avoid over-fitting, the distancemeasure x+y is mapped on a reduced set of context models in a certainway (e.g., by quantizing x+y).

In the particular prior art method, the pattern for context sharing ofan 8×8 transform is approximately an upsampled version of that of a 4×4transform. However, this may also disadvantageously fail to capture thedifference in coefficient distributions of different transforms.

In the existing video coding standards, when coding the significance mapof the transform coefficients of 8×8 transforms or larger, one contextis shared among several transform coefficients to reduce the number ofcontexts. Separate methods are used for various transforms on how toshare the contexts. Such designs cannot be easily extended to futurestandards where more transforms may be used.

SUMMARY

These and other drawbacks and disadvantages of the prior art areaddressed by the present principles, which are directed to methods andapparatus for unified significance map coding.

According to an aspect of the present principles, there is provided anapparatus. The apparatus includes a video encoder for encoding transformcoefficients for at least a portion of a picture. The transformcoefficients are obtained using a plurality of transforms. One or morecontext sharing maps are generated for the transform coefficients basedon a unified rule. The one or more context sharing maps are forproviding at least one context that is shared among at least some of thetransform coefficients obtained from at least two different ones of theplurality of transforms.

According to another aspect of the present principles, there is provideda method in a video encoder. The method includes encoding transformcoefficients for at least a portion of a picture. The transformcoefficients are obtained using a plurality of transforms. One or morecontext sharing maps are generated for the transform coefficients basedon a unified rule. The one or more context sharing maps are forproviding at least one context that is shared among at least some of thetransform coefficients obtained from at least two different ones of theplurality of transforms.

According to yet another aspect of the present principles, there isprovided an apparatus. The apparatus includes a video decoder fordecoding transform coefficients for at least a portion of a picture. Thetransform coefficients are obtained using a plurality of transforms. Oneor more context sharing maps are generated for the transformcoefficients based on a unified rule. The one or more context sharingmaps are for providing at least one context that is shared among atleast some of the transform coefficients obtained from at least twodifferent ones of the plurality of transforms.

According to still another aspect of the present principles, there isprovided a method in a video decoder. The method includes decodingtransform coefficients for at least a portion of a picture. Thetransform coefficients are obtained using a plurality of transforms. Oneor more context sharing maps are generated for the transformcoefficients based on a unified rule. The one or more context sharingmaps are for providing at least one context that is shared among atleast some of the transform coefficients obtained from at least twodifferent ones of the plurality of transforms.

These and other aspects, features and advantages of the presentprinciples will become apparent from the following detailed descriptionof exemplary embodiments, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present principles may be better understood in accordance with thefollowing exemplary figures, in which:

FIG. 1 is a diagram showing an example of the significance map encodingprocedure in accordance with the MPEG-4 AVC Standard;

FIG. 2 is a diagram showing an example of using 15 contexts for a syntaxelement significant_coeff_flag for an 8×8 block;

FIG. 3 is a diagram showing an example of using 15 contexts for a syntaxelement significant_coeff_flag for a 16×16 block;

FIG. 4 is a block diagram showing an exemplary video encoder to whichthe present principles may be applied, in accordance with an embodimentof the present principles;

FIG. 5 is a block diagram showing an exemplary video decoder to whichthe present principles may be applied, in accordance with an embodimentof the present principles;

FIG. 6 is a flow diagram showing a conventional method for encoding asignificance map;

FIG. 7 is a flow diagram showing a conventional method for decoding asignificance map;

FIG. 8 is a flow diagram showing an exemplary method for encoding asignificance map, in accordance with an embodiment of the presentprinciples;

FIG. 9 is a flow diagram showing an exemplary method for decoding asignificance map, in accordance with an embodiment of the presentprinciples;

FIG. 10 is a flow diagram showing another method for encoding asignificance map, in accordance with an embodiment of the presentprinciples; and

FIG. 11 is a flow diagram showing another exemplary method for decodinga significance map, in accordance with an embodiment of the presentprinciples.

DETAILED DESCRIPTION

The present principles are directed to methods and apparatus for unifiedsignificance map coding.

The present description illustrates the present principles. It will thusbe appreciated that those skilled in the art will be able to devisevarious arrangements that, although not explicitly described or shownherein, embody the present principles and are included within its spiritand scope.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the presentprinciples and the concepts contributed by the inventor(s) to furtheringthe art, and are to be construed as being without limitation to suchspecifically recited examples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the present principles, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the block diagrams presented herein represent conceptual views ofillustrative circuitry embodying the present principles. Similarly, itwill be appreciated that any flow charts, flow diagrams, statetransition diagrams, pseudocode, and the like represent variousprocesses which may be substantially represented in computer readablemedia and so executed by a computer or processor, whether or not suchcomputer or processor is explicitly shown.

The functions of the various elements shown in the figures may beprovided through the use of dedicated hardware as well as hardwarecapable of executing software in association with appropriate software.When provided by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, explicituse of the term “processor” or “controller” should not be construed torefer exclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (“DSP”)hardware, read-only memory (“ROM”) for storing software, random accessmemory (“RAM”), and non-volatile storage.

Other hardware, conventional and/or custom, may also be included.Similarly, any switches shown in the figures are conceptual only. Theirfunction may be carried out through the operation of program logic,through dedicated logic, through the interaction of program control anddedicated logic, or even manually, the particular technique beingselectable by the implementer as more specifically understood from thecontext.

In the claims hereof, any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementsthat performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Thepresent principles as defined by such claims reside in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. It is thusregarded that any means that can provide those functionalities areequivalent to those shown herein.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Also, as used herein, the words “picture” and “image” are usedinterchangeably and refer to a still image or a picture from a videosequence. As is known, a picture may be a frame or a field.

Additionally, as used herein, the phrases “significant coefficients” and“significant transform coefficients” refer to transform coefficientshaving a nonzero value.

Moreover, as used herein, the phrase “level information” refers to thevalue of a transform coefficient.

For purposes of illustration and description, examples are describedherein in the context of improvements over the video coding expertsgroup (VCEG) key technical area (KTA) software, using the KTA softwareas the baseline for our description and explaining the improvements andextensions beyond the KTA software. However, it is to be appreciatedthat the present principles are not limited solely to the KTA softwareand/or extensions thereof. Given the teachings of the present principlesprovided herein, one of ordinary skill in this and related arts wouldreadily understand that the present principles are equally applicableand would provide at least similar benefits when applied to extensionsof other standards, or when applied and/or incorporated within standardsnot yet developed. It is to be further appreciated that the presentprinciples also apply to video encoders and video decoders that do notconform to standards, but rather confirm to proprietary definitions.

Turning to FIG. 4, an exemplary video encoder to which the presentprinciples may be applied is indicated generally by the referencenumeral 400. The video encoder 400 includes a frame ordering buffer 410having an output in signal communication with a non-inverting input of acombiner 485. An output of the combiner 485 is connected in signalcommunication with a first input of a transformer and quantizer 425. Anoutput of the transformer and quantizer 425 is connected in signalcommunication with a first input of an entropy coder 445 and a firstinput of an inverse transformer and inverse quantizer 450. An output ofthe entropy coder 445 is connected in signal communication with a firstnon-inverting input of a combiner 490. An output of the combiner 490 isconnected in signal communication with a first input of an output buffer435.

A first output of an encoder controller 405 is connected in signalcommunication with a second input of the frame ordering buffer 410, asecond input of the inverse transformer and inverse quantizer 450, aninput of a picture-type decision module 415, a first input of amacroblock-type (MB-type) decision module 420, a second input of anintra prediction module 460, a second input of a deblocking filter 465,a first input of a motion compensator 470, a first input of a motionestimator 475, and a second input of a reference picture buffer 480.

A second output of the encoder controller 405 is connected in signalcommunication with a first input of a Supplemental EnhancementInformation (SEI) inserter 430, a second input of the transformer andquantizer 425, a second input of the entropy coder 445, a second inputof the output buffer 435, and an input of the Sequence Parameter Set(SPS) and Picture Parameter Set (PPS) inserter 440.

An output of the SEI inserter 430 is connected in signal communicationwith a second non-inverting input of the combiner 490.

A first output of the picture-type decision module 415 is connected insignal communication with a third input of the frame ordering buffer410. A second output of the picture-type decision module 415 isconnected in signal communication with a second input of amacroblock-type decision module 420.

An output of the Sequence Parameter Set (SPS) and Picture Parameter Set(PPS) inserter 440 is connected in signal communication with a thirdnon-inverting input of the combiner 490.

An output of the inverse quantizer and inverse transformer 450 isconnected in signal communication with a first non-inverting input of acombiner 419. An output of the combiner 419 is connected in signalcommunication with a first input of the intra prediction module 460 anda first input of the deblocking filter 465. An output of the deblockingfilter 465 is connected in signal communication with a first input of areference picture buffer 480. An output of the reference picture buffer480 is connected in signal communication with a second input of themotion estimator 475 and a third input of the motion compensator 470. Afirst output of the motion estimator 475 is connected in signalcommunication with a second input of the motion compensator 470. Asecond output of the motion estimator 475 is connected in signalcommunication with a third input of the entropy coder 445.

An output of the motion compensator 470 is connected in signalcommunication with a first input of a switch 497. An output of the intraprediction module 160 is connected in signal communication with a secondinput of the switch 197. An output of the macroblock-type decisionmodule 420 is connected in signal communication with a third input ofthe switch 497. The third input of the switch 497 determines whether ornot the “data” input of the switch (as compared to the control input,i.e., the third input) is to be provided by the motion compensator 470or the intra prediction module 460. The output of the switch 497 isconnected in signal communication with a second non-inverting input ofthe combiner 419 and an inverting input of the combiner 485.

A first input of the frame ordering buffer 410 and an input of theencoder controller 405 are available as inputs of the encoder 400, forreceiving an input picture. Moreover, a second input of the SupplementalEnhancement Information (SEI) inserter 430 is available as an input ofthe encoder 400, for receiving metadata. An output of the output buffer435 is available as an output of the encoder 400, for outputting abitstream.

Turning to FIG. 5, an exemplary video decoder to which the presentprinciples may be applied is indicated generally by the referencenumeral 500. The video decoder 500 includes an input buffer 510 havingan output connected in signal communication with a first input of anentropy decoder 545. A first output of the entropy decoder 545 isconnected in signal communication with a first input of an inversetransformer and inverse quantizer 550. An output of the inversetransformer and inverse quantizer 550 is connected in signalcommunication with a second non-inverting input of a combiner 525. Anoutput of the combiner 525 is connected in signal communication with asecond input of a deblocking filter 565 and a first input of an intraprediction module 560. A second output of the deblocking filter 565 isconnected in signal communication with a first input of a referencepicture buffer 580. An output of the reference picture buffer 580 isconnected in signal communication with a second input of a motioncompensator 570.

A second output of the entropy decoder 545 is connected in signalcommunication with a third input of the motion compensator 570, a firstinput of the deblocking filter 565, and a third input of the intrapredictor 560. A third output of the entropy decoder 545 is connected insignal communication with an input of a decoder controller 505. A firstoutput of the decoder controller 505 is connected in signalcommunication with a second input of the entropy decoder 545. A secondoutput of the decoder controller 505 is connected in signalcommunication with a second input of the inverse transformer and inversequantizer 550. A third output of the decoder controller 505 is connectedin signal communication with a third input of the deblocking filter 565.A fourth output of the decoder controller 505 is connected in signalcommunication with a second input of the intra prediction module 560, afirst input of the motion compensator 570, and a second input of thereference picture buffer 580.

An output of the motion compensator 570 is connected in signalcommunication with a first input of a switch 597. An output of the intraprediction module 560 is connected in signal communication with a secondinput of the switch 597. An output of the switch 597 is connected insignal communication with a first non-inverting input of the combiner525.

An input of the input buffer 510 is available as an input of the decoder500, for receiving an input bitstream. A first output of the deblockingfilter 565 is available as an output of the decoder 500, for outputtingan output picture.

As noted above, the present principles are directed to methods andapparatus for unified significance map coding. To consider thecoefficient distributions from different transform sizes, we adaptcontext sharing patterns to the transform size, while keeping a unifiedstructure for all transform sizes for simplicity.

In order to reduce the number of contexts in coding the residual data, afew transform coefficient positions may share one context model. In theMPEG-4 AVC Standard, the context sharing is designed for each transform.

In accordance with an embodiment of the present principles, we propose aunified rule-based approach to design for all transforms on how to sharethe contexts, where the rule is adaptive to the encoder setup.

In accordance with an embodiment of the present principles, we proposeto unify the context sharing for multiple transforms. While stillkeeping the number of context models at a small number, differenttransforms use the same approach to design context sharing amongmultiple coefficient positions. This unifies the context sharing,preparing the next-generation of standards to accommodate moretransforms.

The present principles propose new methods to code the significance map.Different from the prior art where a separate context sharing method ispre-defined for each transform, we provide a method to unify the contextsharing among different transforms. This simplifies the design of anencoder and/or decoder, particularly when there are multiple transforms.

Typical Significance Map Coding

Turning to FIG. 6, a conventional method for encoding a significance mapis indicated generally by the reference numeral 600. The method 600includes a start block 610 that passes control to a function block 620.The function block 620 reads the pre-defined context sharing maps forthe transform, and passes control to a function block 630. The functionblock 630 encodes the significance map, and passes control to an endblock 699. The context sharing maps are pre-defined for varioustransforms, and the sharing patterns may differ forsignificant_coeff_flag or last_significant_coeff_flag.

Turning to FIG. 7, a conventional method for decoding a significance mapis indicated generally by the reference numeral 700. The method 700includes a start block 710 that passes control to a function block 720.The function block 720 reads the pre-defined context sharing maps forthe transform, and passes control to a function block 730. The functionblock 730 decodes the significance map, and passes control to an endblock 799. The decoder uses the corresponding context sharing map todecode the significance map.

Proposed Method—Unify the Context Sharing Map Generation

In the KTA software, macroblock sizes of 32×32 and 64×64 are supported.For 32×32 blocks, in addition to the existing MPEG-4 AVC Standard motionpartition sizes (16×16, 16×8, 8×16, 8×8, 8×4, 4×8, and 4×4), intercoding using 32×32, 32×16 and 16×32 partitions is also enabled. Biggertransforms can better capture the smoother content in high-definitionvideo. For inter pictures, 16×16, 16×8, and 8×16 transforms are used inaddition to 4×4 and 8×8 transforms for the luma components.Specifically, for each motion partition of sizes 16×16, 16×8, and 8×16,transforms of sizes 16×16, 16×8, and 8×16 may be used in addition to the4×4 and 8×8 transforms. For motion partitions bigger than 16×16, a 16×16transform is used in addition to 4×4 and 8×8 transforms.

To encode the significance map of transform coefficients from newlyintroduced transforms (16×8, 8×16, and 16×16), separate context sharingmaps are designed for each transform. For example, the pattern forcontext sharing of a 16×16 transform is approximately an upsampledversion of that of 8×8. Since transforms usually compact energies intothe first coefficients in the scanning order, such a context sharing maynot suit the transform coefficient distribution. Further, such a contextsharing requires storing the map for each transform.

We propose to unify the generation of the context sharing maps tosimplify the encoder and/or decoder design. In one embodiment, weconvert the 2-D transform coefficient block into a 1-D transformcoefficient array according to a scanning order (for example, a zig-zagscanning order). Depending on the transform coefficient position x, weassign a context according to a rule F(x). This rule is consistent forall transforms. For example,

${F(x)} = \{ {\begin{matrix}{x,\ {0 \leq x < N}} \\{N,\ {otherwise}}\end{matrix},} $where N is the number of contexts. For example, when N=15, there are 15contexts.

When we apply this rule to generate the context sharing maps, there isno need to design separate maps for each transform, and it can be easilyextended to multiple transforms. The same rule is known and used at boththe encoder and decoder.

Turning to FIG. 8, an exemplary method for encoding a significance mapis indicated generally by the reference numeral 800. The method 800includes a start block 810 that passes control to a function block 820.The function block 820 generates the context sharing maps for thetransform based on the rule, and passes control to a function block 830.The function block 830 encodes the significance map, and passes controlto an end block 899. Regarding function block 820, the rule forgenerating the context sharing maps for the transform may involve, forexample, assigning a separate context to the first N coefficientpositions in a pre-defined scanning order (for example, a zig-zagscanning order), and having all other coefficient positions share onecontext.

Turning to FIG. 9, an exemplary method for decoding a significance mapis indicated generally by the reference numeral 900. The method 900includes a start block 910 that passes control to a function block 920.The function block 920 generates the context sharing maps for thetransform based on the rule, and passes control to a function block 930.The function block 930 decodes the significance map, and passes controlto an end block 999. Regarding function block 920, the rule forgenerating the context sharing maps for the transform is the same aswhat is used at the encoder. It may involve, for example, assigning aseparate context to the first N coefficient positions in a pre-definedscanning order (for example, a zig-zag scanning order), and having allother coefficient positions share one context.

Variation

In an embodiment of the present principles, the rule for generating thecontext sharing maps might vary for different sequences, pictureresolutions, quantization parameters, and so forth. For example, we canhave a variation of F(x) as follows:

${F(x)} = \{ {\begin{matrix}{x,} & {0 \leq x < N} \\{{\frac{x - N}{2} + N},} & {x < M} \\{\frac{x + N}{2},} & {otherwise}\end{matrix}.} $

We propose to indicate which rule to use in the syntax. In oneembodiment, the rules are known at both the encoder and the decoder. Insuch a case, the encoder indicates which rule to use through an index inthe bitstream. This approach provides more flexibility.

Turning to FIG. 10, another method for encoding a significance map isindicated generally by the reference numeral 1000. The method 1000includes a start block 1010 that passes control to a function block1020. The function block 1020 decides the rule to generate the contextsharing map, indicates the rule in a bitstream, and passes control to afunction block 1030. The function block 1030 generates the contextsharing maps for the transform based on the rule, and passes control toa function block 1040. Regarding function block 1020, the rule may beselected based on video sequences to which the coefficients correspond,picture resolutions, quantization parameters, and so forth. Regardingfunction block 1030, the rule for generating the context sharing mapsfor the transform may involve, for example, assigning a separate contextfor the first N coefficient positions in a pre-defined scanning order,and having all other coefficient positions share one context.

Turning to FIG. 11, another exemplary method for decoding a significancemap is indicated generally by the reference numeral 1100. The method1100 includes a start block 1110 that passes control to a function block1120. The function block 1120 decodes the rule to generate the contextsharing map, and passes control to a function block 1130. The functionblock 1130 generates the context sharing maps for the transform based onthe rule, and passes control to a function block 1140. The functionblock 1140 decodes the significance map, and passes control to an endblock 1199. Regarding function block 1130, the rule for generating thecontext sharing maps for the transform may involve, for example,assigning a separate context for the first N coefficient positions in apredefined scanning order, and having all other coefficient positionsshare one context.

Syntax

Our proposed method provides the flexibility to select the rule forgenerating the context sharing maps through, for example, the sequenceparameter set (SPS) or picture parameter set (PPS). TABLE 1 showsexemplary syntax for use in a picture parameter set, in accordance withan embodiment of the present principles. Similar syntax can be appliedon other syntax levels, including but not limited to the sequenceparameter set.

TABLE 1 C Descriptor pic_parameter_set_rbsp( ) { ...significance_map_context_rule 1 ae(v) ... }

The semantics of the syntax element shown in TABLE 1 is as follows:

significance_map_context_rule specifies the particular rule to beapplied to generate the context sharing maps for coding the significancemap.

A description will now be given of some of the many attendantadvantages/features of the present invention, some of which have beenmentioned above. For example, one advantage/feature is an apparatushaving a video encoder for encoding transform coefficients for at leasta portion of a picture. The transform coefficients are obtained using aplurality of transforms. One or more context sharing maps are generatedfor the transform coefficients based on a unified rule. The one or morecontext sharing maps are for providing at least one context that isshared among at least some of the transform coefficients obtained fromat least two different ones of the plurality of transforms.

Another advantage/feature is the apparatus having the video encoder asdescribed above, wherein the transform coefficients are two-dimensionaltransform coefficients, and the unified rule specifies mapping thetwo-dimensional transform coefficients into a one-dimensional arrayaccording to a scanning order, and assigning first N coefficients fromamong the transform coefficients to separate contexts.

Yet another advantage/feature is the apparatus having the video encoderwherein the transform coefficients are two-dimensional transformcoefficients, and the unified rule specifies mapping the two-dimensionaltransform coefficients into a one-dimensional array according to ascanning order, and assigning first N coefficients from among thetransform coefficients to separate contexts as described above, whereinremaining coefficients from among the transform coefficients areassigned to a single context.

Moreover, another advantage/feature is the apparatus having the videoencoder wherein the transform coefficients are two-dimensional transformcoefficients, and the unified rule specifies mapping the two-dimensionaltransform coefficients into a one-dimensional array according to ascanning order, and assigning first N coefficients from among thetransform coefficients to separate contexts as described above, whereinthe scanning order is a zig-zag scanning order.

Further, another advantage/feature is the apparatus having the videoencoder as described above, wherein the transform coefficients areencoded into a resultant bitstream, multiple rules are used ascandidates for the unified rule, and the unified rule is indicated inthe resultant bitstream.

Also, another advantage/feature is the apparatus having the videoencoder as described above, wherein the unified rule includes at leasttwo unified rules that vary based on video sequences to which thetransform coefficients correspond, picture resolutions, and quantizationparameters.

These and other features and advantages of the present principles may bereadily ascertained by one of ordinary skill in the pertinent art basedon the teachings herein. It is to be understood that the teachings ofthe present principles may be implemented in various forms of hardware,software, firmware, special purpose processors, or combinations thereof.

Most preferably, the teachings of the present principles are implementedas a combination of hardware and software. Moreover, the software may beimplemented as an application program tangibly embodied on a programstorage unit. The application program may be uploaded to, and executedby, a machine comprising any suitable architecture. Preferably, themachine is implemented on a computer platform having hardware such asone or more central processing units (“CPU”), a random access memory(“RAM”), and input/output (“I/O”) interfaces. The computer platform mayalso include an operating system and microinstruction code. The variousprocesses and functions described herein may be either part of themicroinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU. In addition,various other peripheral units may be connected to the computer platformsuch as an additional data storage unit and a printing unit.

It is to be further understood that, because some of the constituentsystem components and methods depicted in the accompanying drawings arepreferably implemented in software, the actual connections between thesystem components or the process function blocks may differ dependingupon the manner in which the present principles are programmed. Giventhe teachings herein, one of ordinary skill in the pertinent art will beable to contemplate these and similar implementations or configurationsof the present principles.

Although the illustrative embodiments have been described herein withreference to the accompanying drawings, it is to be understood that thepresent principles is not limited to those precise embodiments, and thatvarious changes and modifications may be effected therein by one ofordinary skill in the pertinent art without departing from the scope orspirit of the present principles. All such changes and modifications areintended to be included within the scope of the present principles asset forth in the appended claims.

The invention claimed is:
 1. An apparatus, comprising: a video encoder(400) for transform coefficients obtained using a plurality oftransforms and converted from a two dimensional to a one-dimensionalarray, wherein one or more context sharing maps are generated for thetransform coefficients based on a unified rule for generating thecontext sharing maps, the one or more context sharing maps for providingat least one context that is shared among at least some of the transformcoefficients obtained from at least two different ones of the pluralityof transforms, wherein a context is assigned based on transformcoefficient position.
 2. The apparatus of claim 1, wherein the transformcoefficients are two-dimensional transform coefficients, and the unifiedrule specifies mapping the two-dimensional transform coefficients into aone-dimensional array according to a scanning order, and assigning firstN coefficients from among the transform coefficients to separatecontexts.
 3. The apparatus of claim 1, wherein the transformcoefficients are encoded into a resultant bitstream, multiple rules areused as candidates for the unified rule, and the unified rule isindicated in the resultant bitstream.
 4. The apparatus of claim 1,wherein the unified rule comprises at least two unified rules that varybased on video sequences to which the transform coefficients correspond,picture resolutions, and quantization parameters.
 5. A method,comprising: encoding transform coefficients obtained using a pluralityof transforms and converted from a two dimensional to a one-dimensionalarray, wherein one or more context sharing maps are generated for thetransform coefficients based on a unified rule for generating thecontext sharing maps, the one or more context sharing maps for providingat least one context that is shared among at least some of the transformcoefficients obtained from at least two different ones of the pluralityof transforms, wherein a context is assigned based on transformcoefficient position (800, 1000).
 6. The method of claim 5, wherein thetransform coefficients are two-dimensional transform coefficients, andthe unified rule specifies mapping the two-dimensional transformcoefficients into a one-dimensional array according to a scanning order,and assigning first N coefficients from among the transform coefficientsto separate contexts (820).
 7. The method of claim 6, wherein remainingcoefficients from among the transform coefficients are assigned to asingle context (820).
 8. The method of claim 5, wherein the transformcoefficients are encoded into a resultant bitstream, multiple rules areused as candidates for the unified rule, and the unified rule isindicated in the resultant bitstream (1020).
 9. The method of claim 5,wherein the unified rule comprises at least two unified rules that varybased on video sequences to which the transform coefficients correspond,picture resolutions, and quantization parameters (1020).
 10. Anapparatus, comprising: a video decoder (500) for decoding transformcoefficients obtained using a plurality of transforms and converted froma two dimensional to a one-dimensional array, wherein one or morecontext sharing maps are generated for the transform coefficients basedon a unified rule for generating the context sharing maps, the one ormore context sharing maps for providing at least one context that isshared among at least some of the transform coefficients obtained fromat least two different ones of the plurality of transforms, wherein acontext is assigned based on transform coefficient position.
 11. Theapparatus of claim 10, wherein the transform coefficients aretwo-dimensional transform coefficients, and the unified rule specifiesmapping the two-dimensional transform coefficients into aone-dimensional array according to a scanning order, and assigning firstN coefficients from among the transform coefficients to separatecontexts.
 12. The apparatus of claim 11, wherein remaining coefficientsfrom among the transform coefficients are assigned to a single context.13. The apparatus of claim 10, wherein the transform coefficients aredecoded from a bitstream, multiple rules are used as candidates for theunified rule, and the unified rule is determined from the bitstream. 14.The apparatus of claim 10, wherein the unified rule comprises at leasttwo unified rules that vary based on video sequences to which thetransform coefficients correspond, picture resolutions, and quantizationparameters.
 15. A method, comprising: decoding transform coefficientsfor at least a portion of a picture, the transform coefficients obtainedusing a plurality of transforms and converted from a two dimensional toa one-dimensional array, wherein one or more context sharing maps aregenerated for the transform coefficients based on a unified rule forgenerating the context sharing maps, the one or more context sharingmaps for providing at least one context that is shared among at leastsome of the transform coefficients obtained from at least two differentones of the plurality of transforms, wherein a context is assigned basedon transform coefficient position (900, 1100).
 16. The method of claim15, wherein the transform coefficients are two-dimensional transformcoefficients, and the unified rule specifies mapping the two-dimensionaltransform coefficients into a one-dimensional array according to ascanning order, and assigning first N coefficients from among thetransform coefficients to separate contexts (920).
 17. The method ofclaim 16, wherein remaining coefficients from among the transformcoefficients are assigned to a single context (920).
 18. The method ofclaim 15, wherein the transform coefficients are decoded from abitstream, multiple rules are used as candidates for the unified rule,and the unified rule is determined from the bitstream (1120).
 19. Themethod of claim 15, wherein the unified rule comprises at least twounified rules that vary based on video sequences to which the transformcoefficients correspond, picture resolutions, and quantizationparameters (1120).
 20. A non-transitory computer readable storage mediahaving video signal data encoded thereupon, comprising: transformcoefficients obtained using a plurality of transforms and converted froma two dimensional to a one-dimensional array, wherein one or morecontext sharing maps are generated for the transform coefficients basedon a unified rule for generating the context sharing maps, the one ormore context sharing maps for providing at least one context that isshared among at least some of the transform coefficients obtained fromat least two different ones of the plurality of transforms, wherein acontext is assigned based on transform coefficient position.